Fellow Towdawgs,
As a volunteer tow pilot for High Flights, I have been asked to write a little on how to start the Cub engine. I thought I'd go a little longer winded (typical me) than just how to start the cub and cover some basic operating techniques that I would recommend. You might find these techniques as valuable as what I'm being paid to write these down. [Editor’s note: but Ian has worked with lots of engines, and has thought through some of the issues and considerations in great detail. So I recommend reading all!]
Ask 10 pilots an opinion about anything, and you're gonna get 10 different answers, with all the pilots absolutely sure their answer is the best. My answers come from a maintenance background, from working on airplanes since high school (which I hate to admit how long ago that was!) and from owning old airplanes since the mid 1980's. That does not mean my answers are the only answers, or the only correct answers. It only means that my answers are from my experiences and my background. They are what works for me.
When I arrive at the Cub to start the day, it is often on a Saturday. Saturday's mean the cub hasn't been run for at least a week, maybe many weeks. This is the hardest start for the engine. It is this start that is the most wearing on the engine. In the Warbirds world, where an engine may easily cost upwards of $150k or more, engines are something to be paid attention to.
When an engine has not run for a short while, like a day or so, or like in our case a week or more, it may not have much oil on the internals of the engine. The crank and the cam has little to no oil, the rockers may be dry too. You have metal to metal with little to no lubrication. This is the very last place you want to be hard on an engine during the start. Expensive warbird owners realize that this is very hard on any engine too. That is why you will find that on most all warbirds they will be modified with a pre-oiler. What a pre-oiler does is to circulate oil throughout the internals of the engine just like it does when it is running, before there are any movements of all those cold pieces of metal on metal. Unfortunately we don't have that option on the cub. Therefore, when you go to start it, or any other engine you care about, you keep it turning as slow as possible, certainly at least until you have oil pressure indicated (which should be the first thing you are looking for).
I start the cub by a few pumps of the throttle just seconds before I hit the starter button. The colder it is, the more pumps I give it. Three pumps is usually my max, even on a cold morning, and one pump is usually my minimum. DO NOT CRANK MORE THAN 5 SECONDS. If it hasn't fired in that time, you need to stop cranking and maybe if it's cold, give it a few more pumps of the throttle. Don't overdo the throttle pumping. You are squirting raw gas into the carburetor and if you have some kind of backfire (which is more likely to happen when the throttle is above idle), you can start the engine compartment (or whatever has been soaked in gas) on fire. If that happens, then you should be cranking for as long as you can (now disregard the 5 second concept) to make sure the carb fire is being sucked into the engine where it should be. At least that is what you hope for if it does catch fire. THE THROTTLE SHOULD BE ON THE IDLE STOP POSITION DURING START. The engine will start at the least rpm, and I should be looking for oil pressure at this point. If the engine has a little difficulty running at this point (after I have oil pressure) I may push the throttle up just a little bit to about 600 rpm. Years ago when I was flying Cessna 421's, I would start it with the alternators off too. I would start the engine at a nice stable low rpm. As the oil began to circulate and indicate on the gage, the rpm would pick up slightly. That tells you the engine had lots of friction when first started due to lack of oil lubrication. Then once I got oil pressure stabilized, I would then turn on the alternators. The engines would then drop slightly in rpm. That tells you it's taking energy to turn those alternators. Ideally, I take my time before I turn on that stuff in the cub, just to make it a little easier on it when it first starts.
This is the good time to then put on your headset (please do NOT start the airplane with your headset already on), look up to see that you have a charge indicated on the ammeter and turn on avionics, put on your seatbelt, etc etc. Sometimes I will check to see that the mag switch actually kills both mags when I turn it off. It just takes a second to see that the engine does not run and then I turn the mags back to both. It is probably a better technique to check the mag switch prior to shut down, which would be more logical because it could fail during the next running or flight. I definitely try to make the mag switch check after the last flight of the day as I shut it down for the last time. This is also a time when you want the thermal change as gradual as possible. The engine is built of different metals that expand and contract at different rates. It is designed to operate most efficiently at operating temperature. However, you want the temperature changes to occur as slowly as is possible. More on that later.
Now that you're ready to begin taxing, I recommend that since you hopefully didn't push the airplane out to the most sandy and rocky starting point where the prop will pick up all sorts of damage, you now try to get the job done using as little power (burning gas$) and as little brake (wear and tear $) as possible. Try not to go above 1000 rpm until the engine and oil has a few minutes to begin warming. If you find yourself using lots of brakes, you probably just used too much gas. I personally don't fly the airplane prior to towing, but that is just me. I do recommend that you get the oil temperature into the green band before you fly it though. The oil flows (thereby lubricating and cooling) better when it is a little warmer.
After the cub has flown, and I am getting ready for another tow after shutting it down, I will do one pump of the throttle, bringing it back to the idle stop and then hit the starter (hopefully with the mags on and mixture in ??). If I pump the throttle and then hesitate to hit the starter for any length of time I find it doesn't always start too well. If that is the case, I will stop cranking, do a quick pump of the throttle, and then hit the starter again, never cranking over 5 seconds!
I always start it at idle. In fact if you are not trying to get the airplane to move, you should ALWAYS be at the idle stop. There is no reason to be "idling" the engine on anything but throttle fully back against the idle stop. There are a couple reasons for this. If you are above the lowest idle, you are burning more gas unnecessarily. You are also picking up more rocks and sand damaging the prop, and more than likely you are using more brake to get the airplane stopped. Brakes are what I call negative energy. We all use them, but if you find yourself using them a lot, it might be because you have been creating too much energy (burning too much gas?). If you are not at the lowest idle, you are also creating more heat for the engine. Which is not what you want before a tow with a heavy 2-33 on a warm day. Remember, try to do the job using the least amount of gas and the least amount of brake as efficiently as possible. That's my goal.
Back to temperature changes: since the engine is made of different alloys and is air cooled, it is subject to different expansions and contractions in a potentially short period of time. Front cylinders will cool faster than the rear cylinders too. The quicker the temperature change the harder it is on the engine. Glider towing is very hard on an air cooled engine due to rather quick and radical temperature changes we make the engine perform. Ideally you want to make those temperature changes as smoothly and as slowly as you can. That is hard to do with glider towing! There is always the conundrum of wanting to get back on the ground as soon as you can to get the next glider up and going, but you don't want to shock cool the engine because it is very hard on it. That is why I immediately slip the cub upon glider release. This allows me to keep power (heat) in, but start descending quickly. I pull it back just a bit to maintain about 2500 rpm. After a minute or so I then pull it back to 2400 rpm, and so on, so that when I get to about 1000 feet AFE I will be just above the caution range on the rpm gage. I then take it out of the slip, and bring it through the caution range to about 2000 rpm and I then pull full flaps ( I am usually slowed to flap speed at this point). I then slow the airplane down to near approach speed. I try not to have a lot of speed when the power is low. I feel it adds to the rapid cooling if the power is 2000 or slightly below and speed is above 55 or so. Rapid cooling (or heating) is what we should be trying to avoid. Nobody likes radical changes, especially an air cooled engine!
I told you it would be long winded and opinionated! :))
Happy landings!
Ian